1. Field of the Invention
The present invention relates generally to electronic devices, and more particularly to digital camera modules. Even more particularly, the present invention relates to a system for controlling the optical alignment of a lens assembly with respect an image capture device.
2. Description of the Background Art
Digital camera modules are currently being incorporated into a variety of electronic devices. Such camera hosting devices include, but are not limited to, cellular telephones, personal data assistants (PDAs), and computers. The demand for digital camera modules continues to grow as the ability to incorporate the camera modules into host devices expands. Therefore, one design goal of digital camera modules is to make them as small as possible so that they will fit into an electronic device without substantially increasing the overall size of the device. Means for achieving this goal must, of course, preserve the quality of images captured by the camera modules.
A conventional digital camera module generally includes an image capture device (ICD), a printed circuit substrate (PCB), a housing, and a lens unit. Typically, the components are formed separately and later assembled to create the digital camera module. The assembly typically involves first mounting the ICD on the top surface of the PCB by some suitable adhesive such as, for example, an epoxy. Then, a set of electrical contacts of the ICD are connected to a complementary set of contacts of the PCB by some suitable means (e.g., conductive past, wire bonds, stud bumps, etc.). After the ICD is mounted and electrically connected to the PCB, the housing is mounted on the PCB over the ICD so that the ICD is enclosed within the bottom of the housing. The housing is typically fixed to the PCB via an adhesive such as, for example, epoxy. Once the housing is mounted on the PCB, the lens unit is mounted to the opposite end of the housing to focus incident light onto an image sensor array of the ICD. Typically, the lens unit includes a lens fixed within a lens assembly. The exterior surface of the lens assembly typically defines some type of sloped surface (e.g., threads, cam, ramps, etc.) that engages a complementary sloped surface formed on the housing such that proper focusing can be achieved by rotating the lens unit within the housing. After the lens assembly is properly displaced with respect to the image sensor array, the lens unit is fixed (e.g., via adhesive, thermal weld, etc.) with respect to the housing.
In small digital camera modules, specific details vary depending on the particular design. However, one commonality is that it is critical for the optical axis of the lens assembly to be accurately aligned with respect to the image sensor array. Failing to do so likely results in poor image capture such as, for example, blurred images. Ideally, the lenses should all be coaxially perpendicular to the center of the planar image sensor array. In reality, however, completely eliminating optical tilt in small camera modules is highly unlikely and, therefore, typically only achieved within a predetermined tolerance. Optical tilt commonly refers to the angular error at which the optical axis of the lens assembly deviates from being perpendicular to the image sensor array.
In manufacturing conventional camera modules, several challenges are encountered by manufacturers. For example, to minimize optical tilt in small camera modules several parameters must be controlled during the fabrication and assembly of the camera module components. Such parameters include: planarity and tilt of the top surface of the ICD, planarity and tilt of the bottom surface of the ICD, planarity and tilt of the top surface of the circuit substrate, evenness of the epoxy between the ICD and the top surface of the circuit substrate, planarity and tilt of the bottom surface of the housing, evenness of the epoxy between the top surface of the circuit substrate and the bottom surface of the housing, and tilt of the lens assembly with respect to the housing. Of course, to control such a high number of parameters, camera module manufacturers must implement a high number of delicate and, therefore, expensive processes. Not only does this high number of parameters complicate the overall manufacturing process, but also limits the overall quality of the camera module. Indeed, the overall optical tilt tolerance becomes increasingly more difficult to satisfy as the number of parameters that need to be controlled increases. This is because the overall optical tilt tolerance is defined by an accumulation, or “tolerance stack”, of the parameter tolerances. Even if parameter errors are minimized to the manufacturing limit, they accumulate into a significant overall error because such a high number of parameters have to be controlled. Thus, significant optical tilt and, therefore, poor image quality is oftentimes inevitable in conventional camera modules. This particularly frustrates the further development of such camera modules because, even when minimized, the optical tilt is typically too severe to satisfy the needs of more complex lens systems (e.g., extended depth of field lens systems) and high resolution image capture devices.
What is needed, therefore, is a camera module design that simplifies the optical alignment process. What is also needed is a camera module that can be optically aligned with a higher degree of accuracy. What is also needed is a camera module that can be assembled with more forgiving tolerances. What is also needed is a camera module design that reduces the number of tolerance requirements. What is also needed is a camera module that is less expensive to manufacture.